In a pertussis outbreak in Maine, attack rates were much higher in unvaccinated kids, even though more vaccinated kids got pertussis. While 29 of 214 vaccinated kids got pertussis, a much higher percentage of unvaccinated kids got sick – 6 of 28.

That means your risk of getting pertussis was much higher if you were unvaccinated.

A 2013 pertussis outbreak in Florida is a good example that even with all the bad press it gets, the DTaP and Tdap vaccines work too. This outbreak was started by an unvaccinated child at a charter school with high rates of unvaccinated kids. About 30% of unvaccinated kids got sick, while there was only one case “in a person who reported having received any vaccination against pertussis.”

In another 2013 pertussis outbreak in Florida, this time in a preschool, although most of the kids were vaccinated, the outbreak started with “a 1-year-old vaccine-exempt preschool student.” And the classroom with the highest attack rate, was “one in which a teacher with a laboratory-confirmed case of pertussis who had not received a Tdap booster vaccination, worked throughout her illness.”

Why do so many unvaccinated kids get pertussis these days?

“We found evidence of an increase in exemption rates, spatial clustering of nonmedical exemptions, and space-time clustering of pertussis in Michigan. There was considerable overlap between the clusters of exemptions and the clusters of pertussis cases.”

Omer et al on Geographic Clustering of Nonmedical Exemptions to School Immunization Requirements and Associations With Geographic Clustering of Pertussis

“Children of parents who refuse pertussis immunizations are at high risk for pertussis infection relative to vaccinated children. Herd immunity does not seem to completely protect unvaccinated children from pertussis.”

Glanz et al on Parental refusal of pertussis vaccination is associated with an increased risk of pertussis infection in children.

Another important consideration – in addition to the fact that more unvaccinated kids get pertussis, when they get pertussis, it is more severe than those who are vaccinated.

“Serious pertussis symptoms and complications are less common among age-appropriate number of pertussis vaccines (AAV) pertussis patients, demonstrating that the positive impact of pertussis vaccination extends beyond decreasing risk of disease.”

13 vaccines, including 5 doses of DTaP, 4 doses of IPV (polio), 3 or 4 doses of hepatitis B, 3 or 4 doses of Hib (the number of doses depends on the vaccine brand used), 4 doses of Prevnar, 2 or 3 doses of rotavirus (the number of doses depends on the vaccine brand used), 2 doses of MMR, 2 doses of Varivax (chicken pox), 2 doses of hepatitis A, 1 doses of Tdap, 2 or 3 doses of HPV (the number of doses depends on the age you start the vaccine series), 2 doses of MCV4 (meningococcal vaccine), and yearly influenza vaccines

Can an unvaccinated child really get tetanus after a toe nail injury? Photo by Petrus Rudolf de Jong (CC BY 3.0)

None of it is true.

At age four years, when your preschooler routinely gets their DTaP, IPV, MMR, and chicken pox shots before starting kindergarten, how many vaccines or doses do you think they got? Two, because they got Kinrix or Quadracel (DTaP/IPV combo) and Proquad (MMR/chickenpox combo)? Four, because they got separate shots? Or Eight, because you think you should count each component of each vaccine separately?

Know that even if you do want to count them separately, it really just means that with those two or four shots, your child got protection against eight different vaccine-preventable diseases – diphtheria, tetanus, pertussis, polio, measles, mumps, rubella, and chicken pox.

They also push the myth that more vaccinated than unvaccinated kids get sick in most outbreaks.

Vaccinated vs Unvaccinated in an Outbreak

So are outbreaks usually caused by kids who have been vaccinated?

No, of course not.

Do we sometimes see more vaccinated than unvaccinated kids in some of these outbreaks?

Yes, sometimes we do.

Vaccine Epidemiology

Wait, what?

Yes, we sometimes see more vaccinated than unvaccinated kids in an outbreak.

How can that be if vaccines work?

It is actually very easy to understand once you learn a little math and a little more epidemiology.

Basically, it is because while vaccines work, they don’t work 100% of the time, and more importantly, there are way more vaccinated kids around than unvaccinated kids.

The Mathematics of Disease Outbreaks

That means that you need to understand that more than the absolute number of vaccinated and unvaccinated people that got sick in an outbreak, you really want to know the percentages of vaccinated vs unvaccinated kids who got sick.

For example, in a school with 1,000 kids, you might be very surprised if six kids got a vaccine preventable disease, and three of them were vaccinated, leaving three unvaccinated.

Does that really mean that equal amounts of vaccinated and unvaccinated kids got sick?

I guess technically, but in the practical sense, it only would if half of the kids in the school were unvaccinated. Now unless they go to a Waldorf school, it is much more likely that over 90 to 95% of the kids were vaccinated, in which case, a much higher percentage of unvaccinated kids got sick.

Before we use a real world example, some terms to understand include:

attack rate – how many people will get sick when exposed to a disease

basic reproductive number or Ro – different for each disease, Ro basically tells you just how contagious a disease is and ranges from about 1.5 for flu, 8 for chicken pox, and 15 for measles

vaccine coverage – how many people are vaccinated

vaccine efficacy – how well a vaccine works

You also need to know some formulas:

attack rate = new cases/total in group

vaccine coverage rate = number of people who are fully vaccinated / number of people who are eligible to be vaccinated

Unfortunately, it is often hard to use these formulas in most outbreaks.

Why?

For one thing, it is hard to get accurate information on the vaccination status of all of the people in the outbreak. In addition to those who are confirmed to be vaccinated or unvaccinated, there is often a large number who’s vaccination status is unknown. And even if you know the vaccination status of everyone in the outbreak, it can be even harder to get the vaccine coverage rate or a neighborhood or city.

Outbreaks of Vaccine Preventable Diseases

Reports of measles outbreaks among highly vaccinated populations are from before we started doing a second dose of MMR… way back in 1994.

For example, with measles, it is typically an unvaccinated person who travels out of the country, returns home after they have been exposed but are still in their incubation period, and then exposes others once they get sick. And the great majority of folks in these measles outbreaks are unvaccinated.

Some examples of these outbreaks include:

the 2014 Ohio measles outbreak that started with two unvaccinated Amish men getting measles in the Philippines while on a missionary trip and ended up with at least 388 cases before it was over, almost all unvaccinated

a 2013 North Carolina measles outbreak with 22 cases started after an unvaccinated traveler had returned from India

an outbreak of measles in New York, in 2013, with at least 58 cases, tarted with an intentionally unvaccinated teen returning from a trip to London

a 2011 outbreak of measles in Minnesota, when an unvaccinated child traveled out of the country, developed measles, and returned to his undervaccinated community, causing the state’s largest measles outbreak in 20 years

But what about mumps and pertussis?

Those outbreaks are all among vaccinated kids, right?

Nope.

In one of the biggest mumps outbreak, in Arkansas, only 71% of people were up-to-date on their vaccines!

And keep in mind that while we do know that there are issues with waning immunity with some vaccines, you are still much more likely to become infected and get others sick if you are not vaccinated. And you will likely have a much more severe disease.

A 2013 pertussis outbreak in Florida is a good example that even with all the bad press it gets, the DTaP and Tdap vaccines work too. This outbreak was started by an unvaccinated child at a charter school with high rates of unvaccinated kids. About 30% of unvaccinated kids got sick, while there was only one case “in a person who reported having received any vaccination against pertussis.”

In another 2013 pertussis outbreak in Florida, this time in a preschool, although most of the kids were vaccinated, the outbreak started with “a 1-year-old vaccine-exempt preschool student.” And the classroom with the highest attack rate, was “one in which a teacher with a laboratory-confirmed case of pertussis who had not received a Tdap booster vaccination, worked throughout her illness.”

In outbreak after outbreak, we see the same thing, sometimes with deadly consequences – an unvaccinated child or adult triggers an outbreak and then a lot of unvaccinated folks get sick. Unfortunately, others get caught up in these outbreaks too, including those too young to be vaccinated, those who can’t be vaccinated because of true medical exemptions, and those whose vaccines may not have worked as well as we would have liked.